Surface residual stresses are widely known to have a major effect upon fatigue and stress corrosion performance of metallic parts. Residual stresses, such as tensile residual stresses, add to the applied stresses imposed on a part in service and can lead to more rapid fatigue or stress corrosion failure. Compressive residual stresses have been shown to have the effect of countering applied tension and have been used to generally improve the life of a part by reducing its overall stress state and by retarding fatigue and stress corrosion crack initiation and growth. A variety of surface enhancement methods, such as shot peening, gravity peening, laser shocking, deep rolling, low plasticity burnishing, split sleeve cold expansion and similar mechanical treatments, have been developed to induce a beneficial layer of compressive residual stress along the surface of a part. The depth and magnitude of such residual stress and diffraction peak broadening distributions produced by such surface enhancement treatments are typically measured using x-ray diffraction methods.
Shot peening has been commonly used in industry, particularly in the automotive and aerospace industries, as the preferred method of inducing compressive stress in the surface of a part. During the shot peening process, metallic, glass, or ceramic pellets are projected, mechanically or through air pressure, such that they impinge on the surface of a work piece. The parameters used to shot peen the work piece are selected by determining the time required to achieve a specified “Almen intensity” which is determined from arc heights representing the deflection due to residual stresses induced in a thin standard steel Almen strip. The “coverage” of the shot peening process is determined by examination of the surface of the work piece at magnification to ensure that essentially the entire surface has been impacted at least once by projected pellets. This condition of an entirely impacted surface is defined to be 100% coverage and is achieved by shot peening using fixed peening parameters in a measured time as designated herein as 1T. For a given peening apparatus and peening parameters (including shot size, hardness and flow rate), the shot peening processing time to achieve a fixed percent coverage is commonly taken as proportional to the time required to achieve 100% coverage.
Until now, it has been believed that the surface of the work piece must be essentially entirely impacted by shot (i.e. entirely covered by impact craters or dimples) during the shot peening process and shot to at least 100% coverage in order to achieve a consistent and desirable depth and magnitude of residual compression. Indeed, many military and industrial shot peening standards recommend shot peening to a minimum of 100% coverage, and often require 125% to 200% coverage, in order to achieve reliable fatigue and stress corrosion life improvement. Most of the published fatigue data supporting the 100% minimum coverage has been developed using fully reversed axial loading or bending with a stress ratio (R=Smin/Smax) of −1.0.
Unfortunately, it has been shown that such conventional shot peening induces a high degree of surface deformation and cold working which increases with increasing shot peening coverage. This relatively large amount of cold working leaves the surface susceptible to rapid thermal relaxation. Further, such cold working has also been found to increase the yield strength of the surface and leaves the residual stress layer within the surface susceptible to mechanical relaxation in the event of deformation following shot peening.
Accordingly, a need exists for a method for shot peening the surface of a part to induce a layer of residual compressive stress therein to improve the part's fatigue and stress corrosion performance and also renders the surface less susceptible to thermal and mechanical relaxation than parts treated by convention shot peening.